1. Field of the Invention
The present invention relates to a method and a device for measuring a thickness of a birefringent body having a birefringent index An that is uniform in the thickness direction, such as an alignment-treated liquid crystal layer included in a liquid crystal display element.
2. Description of the Background Art
Liquid crystal display elements are utilized as displays in significantly wide-ranging areas, since they have small power consumption and small size, and are lightweight. In recent years, color liquid crystal display elements of a reflection-type have, among others, come into practical use, and the demand therefor has rapidly expanded.
A reflection-type color liquid crystal display, a conventional reflection-type liquid crystal display and a conventional transmission-type liquid crystal display are herein described for their respective characteristics. A conventional transmission-type liquid crystal display 111 includes, as shown in FIG. 20, a liquid crystal cell 100 constituted by a pair of transparent substrates 12 with outer sides interposed by polarizing plates 14 or elliptical polarizing plates. A voltage applied to a liquid crystal layer 11 changes an alignment of liquid crystal molecules and controls an polarization state of light transmitting through the liquid crystal layer. The light transmits through polarizing plate 14 of an exit side, with the controlled polarization state, so that a difference in brightness is generated in accordance with the polarization state, resulting in a desired display. Further, a conventional transmission-type liquid crystal display 112 also uses, as shown in FIG. 21, liquid crystal cell 100 which is basically the same as that of the transmission-type, and a reflector 15 is further attached to an outer side of polarizing plate 14 of a backside, for displaying.
However, along with finer picture elements attained in recent years, a conventional method in which reflector 15 is attached to the outside for displaying on conventional reflection-type liquid crystal display 112 have caused a problem in that an image is doubled due to a parallax generated by the thickness of substrate 12 and polarizing plate 14 between reflector 15 and liquid crystal layer 11. Particularly when it comes to a color display, the doubled image causes color mixture, which significantly lowers a display quality. Further, when the color display is performed using a micro color filter, light entered at an angle passes through color filters of different colors at an incoming side and an outgoing side, lowering color saturation and thereby greatly degrading color reproduction property.
To solve this problem, a configuration is proposed, as shown in FIG. 22, in which reflector 15 is disposed within liquid crystal cell 101. Because this configuration can suppress the doubled image due to the parallax and degrading of the color reproduction, reflection-type color liquid crystal display 113 mainly uses such a configuration in which a reflector is disposed within liquid crystal cell 101. In reflection-type color liquid crystal display 113, light is reflected by reflector 15 within liquid crystal cell 101, which is different from conventional reflection-type liquid crystal display 112 in which light transmitted through liquid crystal cell 100 is reflected after it completely exits from the liquid crystal cell.
Further, in reflection-type liquid crystal displays 112 and 113, light passes through the same liquid crystal layer 11 twice, so that the thickness of liquid crystal layer 11 has greater effects on the display quality, compared to that of a transmission-type liquid crystal display. This is because a magnitude of a change provided by a member having birefringence to the polarization state of the light entered thereto is proportional to the birefringent index (also referred to as xe2x80x9crefractive index anisotropyxe2x80x9d) and the thickness of the member.
Thus, measurement of the thickness of liquid crystal layer 11, i.e. a so-called xe2x80x9ccell gap,xe2x80x9d for measuring the thickness and uniformity has more significant meaning than the case with the conventional transmission-type liquid crystal display, in order to maintain the display quality of the reflection-type color liquid crystal display.
A conventional technology for measuring a thickness of a liquid crystal layer (hereinafter referred to as xe2x80x9ccell gapxe2x80x9d) is a measurement method disclosed in Japanese Patent Laid-Open No. 4-307312 (hereinafter referred to as xe2x80x9cconventional method 1xe2x80x9d). In the conventional method 1, light enters into a liquid crystal cell via a polarizer arranged in a direction which is +45xc2x0 rotated from an allignment direction of an entrance light side of the liquid crystal cell. Further, exit light transmitted through the liquid crystal cell exits via a polarizer arranged in a direction which is +45xc2x0 rotated from an alignment direction of an exit light side of the liquid crystal cell, to measure an intensity of the exit light. An operation is performed from a value of a wavelength at which the measured exit light intensity assumes a maximal value or a minimal value, to find a value of a cell gap. In the conventional method 1, the operation is performed using
xcex94nxc2x7d=xcexoxc2x7(mo2xe2x88x92"THgr"2/xcfx802)1/2 
as a condition of the maximal intensity, and
xcex94nxc2x7d=xcexo((moxe2x88x92xc2xd)2xe2x88x92"THgr"2/xcfx802)1/2 
as a condition of the minimal intensity. The respective variables represent the values below.
xcex94n: birefringent index (xcex94n=extra ordinary index nexe2x88x92ordinary index no)
xcexo: wavelength to be maximum or minimum
mo: degree
"THgr": twist angle of the liquid crystal layer
An example of a conventional technology for measuring the cell gap by utilizing reflected light is a measurement method disclosed in Japanese Patent Laid-Open No. 10-232113 (hereinafter referred to as xe2x80x9cconventional method 2xe2x80x9d). In the conventional method 2, mercury lamp light is projected on a substrate to fluorescence-excite alignment films for liquid crystal alignment formed at inner sides of upper and lower substrates, and images are formed for the radiance on a CCD (Charge-Coupled Device) by a lens to measure the cell gap by calculating a distance between the alignment films of the upper and lower substrates from the distance between the images.
The conditional equations by which the exit light intensity is maximum or minimum, used in the conventional method 1, are the equations used only for the transmission-type liquid crystal cell. That is, it is assumed that the entered light passes through the liquid crystal layer only once during which the light is polarized. In the reflection-type liquid crystal cell, however, the light passed through the liquid crystal layer and was polarized is reflected at the reflector, and again passes through the liquid crystal layer for further polarization before exiting. Therefore, the operation for the conventional method 1 cannot be applied, as it is, to the reflection-type liquid crystal cell.
Further, there may be a reflection-type color liquid crystal display, in which a portion of the reflector not used for display, for example, a portion corresponding to a gap between electrodes, is removed by etching or the like. In such a case, the measurement of the cell gap appears to be possible by the conventional method 1, by utilizing transmitted light leaking from the portion where a reflection layer is removed. However, etching of the reflector generates a step in the liquid crystal layer between an etched portion and a nonetched portion, resulting in a possible value difference between the etched portion and a picture element portion used for display. Thus, there is a need, after all, for measuring the cell gap of the picture element portion. Thus, it is required to use the reflection light rather than the transmission light to measure the cell gap.
The conventional method 2 in which the cell gap is measured by using the reflected light is described in Japanese Patent Laid-Open No. 10-232113, such that a liquid crystal cell to which liquid crystal is not yet injected is particularly to be measured. The conventional method 2 forms an image of fluorescence generated from an alignment agent on a CCD sensor, assuming that no liquid crystal exists within the liquid crystal cell, so that the cell gap cannot be measured after the injection of the liquid crystal cell.
However, the cell gap is greatly changed after the injection to the liquid crystal cell, depending on temperature at the time of injection, leaving time after completion of injection, viscosity of liquid crystal, and amount of dispersion of a spacer placed in a gap of substrates and its particle diameter. Therefore, the cell gap measurement only for the non-injected liquid crystal cell cannot perform sufficient quality control of the liquid crystal cell. It is essentially desirable to control the cell gap after liquid crystal injection in order to control the quality of the liquid crystal cell.
Thus, an object of the present invention is to provide a method and a device for measuring a thickness of a birefringent body having a birefringent index xcex94n which is uniform in the thickness direction, such as an alignment treated liquid crystal layer, using reflected light rather than transmitted light.
To achieve the object described above, according to an aspect of the present invention, a method of measuring a thickness d of a liquid crystal layer having alignment-treated upper and lower surfaces and a birefringent index xcex94n which is uniform in the thickness direction, in a reflection-type liquid crystal display element including the liquid crystal layer between a pair of substrates and having a reflection region at least in a part of one of the substrates, includes a light-receiving step of entering light from a light source into the liquid crystal layer via a first polarizing means and receiving, via a second polarizing means, reflected light exited from the liquid crystal layer by reflecting at the reflection region; a dispersing step of spectrally resolving the reflected light received by the light-receiving means to detect a relation between a wavelength xcex and a reflected light intensity; a wavelength deriving step of finding a wavelength satisfying a polarizing plane-maintaining condition in that the reflected light returns maintaining a same polarizing plane as a polarizing plane at the time of the entering, i.e., that a difference in optical path lengths between an ordinary ray and an extraordinary ray of the reflected light is a sum of an integer multiple of a wavelength and a half-wavelength, or an integer multiple of a wavelength; xcex94nxc2x7d deriving step of finding a reasonable xcex94nxc2x7d from the wavelength found by the wavelength deriving step and a known twist angle of the liquid crystal layer to find a relation between the wavelength and xcex94nxc2x7d from a plurality of combinations of the wavelength and xcex94nxc2x7d; and a thickness deriving step of finding d by assigning a known combination of wavelength xcex and xcex94n to the relation.
By using the steps described above, the reflected light is utilized rather than transmitted light, so that an accurate measurement can also be performed for a reflection-type liquid crystal cell, irrespective of presence/absence of the transmitted light. Further, the steps are for detecting a change of the polarization state and no imaging of the reflected light is required, so that the thickness measurement can be performed irrespective of the property of the reflection region, such as being specular and diffusible.
Preferably, the wavelength deriving step is performed by finding a value of a wavelength at which the reflected light intensity assumes an extreme value. By using the step, the wavelength that satisfies the polarizing plane-maintaining condition can easily be found from a spectrum, and thus the measurement can readily be performed.
More preferably, a Jones matrix is used in the xcex94nxc2x7d deriving step. By employing this step, the variation of the polarization state can be expressed only by applying a mechanical calculation, and thus the operation for obtaining thickness d can be simplified.
More preferably, in the case that xcex2/xcfx80 is n or n+{fraction (1/2)} (n is an integer) and xcex1=xcex94nxc2x7dxcfx80/"THgr"xcex . . . (equation 1) and xcex2="THgr"xc2x7(1+xcex12)1/2 . . . (equation 2) in which "THgr" is a known twist angle of the liquid crystal layer, a reasonable value of xcex2/xcfx80 is found from wavelength xcex when a polarizing plane of the reflected light is maintained, the twist angle "THgr", and the equations 1 and 2, and a relation between the wavelength and xcex94nxc2x7d is found by a calculation from the obtained value of xcex2/xcfx80.
By employing the steps described above, unreasonable values of xcex2/xcfx80 are eliminated, such that an approximation representing the most reasonable relation between the wavelength and xcex94nxc2x7d can be found.
Preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are orthogonal to each other. By using this step, the spectrum of the exit light will be a minimal value of 0 at the wavelength satisfying the polarizing plane-maintaining condition, which facilitates finding of the wavelength.
Preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are parallel to each other. By using this step, the spectrum of the exit light will be a maximal value at the wavelength satisfying the polarizing plane-maintaining condition, which facilitates finding of the wavelength.
Preferably, when an angle formed by the transmission axis of the first polarizing means and a direction of alignment on a plane of the liquid crystal layer contacting a substrate to which the light enters is assumed to be xcfx86, the light-receiving step, the dispersing step and the wavelength deriving step are performed for a plurality of xcfx86s in a range of 0xc2x0 to 90xc2x0. By employing these steps, the values of xcex2xcfx80 can be obtained for a plurality of wavelengths xcex by a plurality of xcfx86s, and thus the measurement can accurately be performed.
Preferably, a Cauchy dispersion formula is used in the xcex94nxc2x7d deriving step. By using this step, an approximation of wavelength xcex and xcex94nxc2x7d can be obtained even for the measurement only for one xcfx86, and thus the thickness measurement can be simplified.
According to another aspect of the present invention, the reflection region has a diffusibility and light is received by the light-receiving means at a position off a positive reflection direction corresponding to the entering. By employing this step, a positive reflection component from an upper substrate can be excluded, so that the thickness measurement can be performed also for the reflection-type liquid crystal cell using the reflector having diffusibility, without any effect caused by the positive reflection component from the upper substrate.
According to one aspect of the present invention, a device for measuring a thickness includes a light source; a first polarizing means for transmitting light from the light source; a second polarizing means for transmitting reflected light reflected at an object to be measured; a light receiving means for receiving the reflected light transmitted through the second polarizing means; a dispersing means for spectrally resolving the reflected light received by the light-receiving means to detect a relation between a wavelength xcex and a reflected light intensity; a wavelength deriving means for finding a wavelength satisfying a polarizing plane maintaining condition in that the reflected light returns maintaining a same polarizing plane as a polarizing plane at the time of the entering, i.e. that a difference in optical path lengths between an ordinary ray and an extraordinary ray of the reflected light is a sum of an integer multiple of the wavelength and a half-wavelength, or an integer multiple of a wavelength; a xcex94nxc2x7d deriving means for finding a reasonable xcex94nxc2x7d from the wavelength found by the wavelength deriving means and a known twist angle of the liquid crystal layer to find a relation between the wavelength and xcex94nxc2x7d from a plurality of combinations of the wavelength and xcex94nxc2x7d; and a thickness deriving means for finding d by assigning a known combination of wavelength xcex and xcex94n to the relation.
By using the configuration described above, the reflected light is utilized rather than transmitted light, so that an accurate measurement can also be performed for a reflection-type liquid crystal cell. Further, detection is performed for a change of the polarization state, so that the thickness measurement can be performed irrespective of the property of the reflection region, such as being specular and diffusible.
More preferably, the wavelength deriving means is performed by finding a value of a wavelength at which the reflected light intensity assumes an extreme value. By using the configuration, the wavelength that satisfies the polarizing plane-maintaining condition can easily be found from a spectrum, and thus the measurement can readily be performed.
More preferably, a Jones matrix is used in the xcex94nxc2x7d deriving means. By employing this configuration, the variation of the polarization state can be expressed only by applying a mechanical calculation, and thus the operation for obtaining thickness d can be simplified.
More preferably, in the case that xcex2/xcfx80 is n or n+xc2xd (n is an integer) and xcex1=nxc2x7dxcfx80/"THgr"xcex (equation 1) and xcex2="THgr"xc2x7(1+xcex12)1/2 . . . (equation 2) in which "THgr" is a known twist angle of the liquid crystal layer, a reasonable value of xcex2/xcfx80 is found from wavelength xcex when a polarizing plane of the reflected light is maintained, the twist angle "THgr", and the equations 1 and 2, and a relation between the wavelength and xcex94nxc2x7d is found by a calculation from the obtained value of xcex2/xcfx80.
By employing the configuration, unreasonable values of xcex2/xcfx80 are eliminated, such that an approximation representing the most reasonable relation between the wavelength and xcex94nxc2x7d can be found.
More preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are orthogonal to each other. By this configuration, the spectrum of the exit light will be a minimal value of 0 at the wavelength satisfying the polarizing plane maintaining condition, which facilitates finding of the wavelength.
More preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are parallel to each other. By this configuration, the spectrum of the exit light will be a maximal value at the wavelength satisfying the polarizing plane-maintaining condition, which facilitates finding of the wavelength.
More preferably, when an angle formed by the transmission axis of the first polarizing means and a direction of alignment on a plane of the liquid crystal layer contacting a substrate to which the light enters is assumed to be xcfx86, the light receiving means, the dispersing means and the wavelength deriving means are used for a plurality of xcfx86s in a range of 0xc2x0 to 90xc2x0. By employing the configuration, the values of xcex2/xcfx80 can be obtained for a plurality of wavelengths xcex by a plurality of xcfx86s, and thus the measurement can accurately be performed.
Preferably, the xcex94nxc2x7d deriving step uses a Cauchy dispersion formula. By using the configuration, an approximation of wavelength xcex and xcex94nxc2x7d can be obtained even for the measurement only for one xcfx86, and thus the thickness measurement can be simplified.
According to another aspect of the present invention, the reflection region has a diffusibility and light is received by the light-receiving means at a position off a positive reflection direction corresponding to the entering. By employing this configuration, a positive reflection component from an upper substrate can be excluded, so that the thickness measurement can be performed also for the reflection-type liquid crystal cell using the reflector having diffusibility, without any effect caused by the positive reflection component from the upper substrate.
To achieve the object described above, according to another aspect of the present invention, a method of measuring a thickness d of a liquid crystal layer having alignment-treated upper and lower surfaces and a birefringent index xcex94n which is uniform in the thickness direction, in a reflection-type liquid crystal display element including the liquid crystal layer between a pair of substrates and having a reflection region at least in a part of one of the substrates, includes a light-receiving step of entering light from a monochromatic light source, with a wavelength at which birefringent index xcex94n of liquid crystal is known, into the liquid crystal layer via a first polarizing means and receiving, via a second polarizing means, reflected light exited from the liquid crystal layer by reflecting at the reflection region; a rotational light-receiving step of receiving light while changing a rotational angle which is an angle formed by the first and second polarizing means and the liquid crystal layer when seen from above, maintaining an angle formed by respective transmission axes of the first polarizing means and the second polarizing means to be constant by engaging with the light receiving step; an angle deriving step of finding the rotational angle satisfying a polarizing plane-maintaining condition in that the reflected light returns maintaining a same polarizing plane as a polarizing plane at the time of the entering, i.e., that a difference in optical path lengths between an ordinary ray and an extraordinary ray of the reflected light is a sum of an integer multiple of a wavelength and a half-wavelength, or an integer multiple of a wavelength; and a thickness deriving step of finding d from a reasonable xcex94nxc2x7d selected in accordance with a relation between desired wavelength and xcex94nxc2x7d derived from an angle found by the angle deriving step and a known twist angle of the liquid crystal layer.
By using the steps described above, the reflected light is utilized rather than transmitted light, so that an accurate measurement can also be performed for a reflection-type liquid crystal cell, irrespective of presence/absence of the transmitted light. Further, the steps are for detecting a change of the polarization state and no imaging of the reflected light is required, so that the thickness measurement can be performed irrespective of the property of the reflection region, such as being specular and diffusible. Furthermore, the monochromatic light source with the wavelength at which birefringent index xcex94n of liquid crystal is known is used, so that xcex94nxc2x7d at the wavelength at which xcex94n is known can directly be obtained without an approximation. Thus, the measurement of d can further be accurately performed without being affected by accuracy of the approximation.
Preferably, the angle deriving step is performed by finding a value of the rotational angle at which the reflected light intensity assumes an extreme value. By using the step, the rotational angle that satisfies the polarizing plane-maintaining condition can easily be found from a variation of intensity, and thus the measurement can readily be performed.
More preferably, a Jones matrix is used to find xcex94nxc2x7d in the thickness deriving step. By employing this step, the variation of the polarization state can be expressed only by applying a mechanical calculation, and thus the operation for obtaining thickness d can be simplified.
More preferably, using the Jones matrix, in the case that an angle formed by a transmission axis of the first polarizing means and a direction of alignment on an entrance side surface of the liquid crystal layer is assumed to be xcfx86, and that a twist angle of the liquid crystal layer is assumed to be "THgr" a reasonable value of xcex94nxc2x7d/xcfx80 is found from an angle xcfx86 at which a polarizing plane of the reflected light is maintained and from the known twist angle "THgr", to find xcex94nxc2x7d at a wavelength xcex from an obtained value of xcex94nxc2x7d/xcex.
By employing the steps described above, unreasonable values of xcex94nxc2x7d/xcfx80 are eliminated such that the most reasonable xcex94nxc2x7d/xcex can be obtained, and xcex94n and xcex are known values, so that d can be found.
Preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are orthogonal to each other. By using this step, the intensity of the exit light will be a minimal value of 0 at the rotational angle satisfying the polarizing plane-maintaining condition, which facilitates finding of the rotational angle.
Preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are parallel to each other. By using this step, the intensity of the exit light will be a maximal value at the rotational angle satisfying the polarizing plane-maintaining condition, which facilitates finding of the rotational angle.
According to another aspect of the present invention, the reflection region has a diffusibility and light is received by the light-receiving means at a position off a positive reflection direction corresponding to the entering. By employing this step, a positive reflection component from an upper substrate can be excluded, so that the thickness measurement can be performed also for the reflection-type liquid crystal cell using the reflector having diffusibility, without any effect caused by the positive reflection component from the upper substrate.
A device for deriving an angle according to the present invention includes a monochromatic light source; a first polarizing means for transmitting light from the monochromatic light source; a second polarizing means for transmitting reflected light reflected at an object to be measured; a light-receiving means for receiving the reflected light transmitted through the second polarizing means; a rotational light-receiving means for receiving light while changing a rotational angle which is an angle formed by the first and second polarizing means and the liquid crystal layer when seen from above, maintaining an angle formed by respective transmission axes of the first polarizing means and the second polarizing means to be constant by engaging with the light receiving means; and an angle deriving means for finding the rotational angle satisfying a polarizing plane-maintaining condition in that the reflected light returns maintaining a same polarizing plane as a polarizing plane at the time of the entering, i.e. that a difference in optical path lengths between an ordinary ray and an extraordinary ray of the reflected light is a sum of an integer multiple of the wavelength and a half-wavelength, or an integer multiple of a wavelength.
By using the configuration described above, the reflected light is utilized rather than transmitted light, so that an accurate measurement can also be performed for a reflection-type liquid crystal cell. Further, detection is performed for a change of the polarization state, so that the rotational angle satisfying the polarizing plane-maintaining condition can be derived irrespective of the property of the reflection region, such as being specular and diffusible. The obtained rotational angle can be utilized for finding a cell gap d.
To achieve the object described above, according to another aspect of the present invention, a device for measuring a thickness includes a monochromatic light source; a first polarizing means for transmitting light from the monochromatic light source; a second polarizing means for transmitting reflected light reflected at an object to be measured; a light receiving means for receiving the reflected light transmitted through the second polarizing means; a rotational light-receiving means for receiving light while changing a rotational angle which is an angle formed by the first and second polarizing means and the liquid crystal layer when seen from above, maintaining an angle formed by respective transmission axes of the first polarizing means and the second polarizing means to be constant by engaging with the light receiving means; an angle deriving means for finding the rotational angle satisfying a polarizing plane-maintaining condition in that the reflected light returns maintaining a same polarizing plane as a polarizing plane at the time of the entering, i.e. that a difference in optical path lengths between an ordinary ray and an extraordinary ray of the reflected light is a sum of an integer multiple of the wavelength and a half-wavelength, or an integer multiple of a wavelength; a xcex94nxc2x7d deriving means for finding a relation between a wavelength xcex and xcex94nxc2x7d from an angle found by the angle deriving means; and a thickness deriving means for finding d by using wavelength xcex and a known xcex94n.
By using the configuration described above, the reflected light is utilized rather than transmitted light, so that an accurate measurement can also be performed for a reflection-type liquid crystal cell. Further, detection is performed for a change of the polarization state, so that the thickness measurement can be performed irrespective of the property of the reflection region, such as being specular and diffusible. Furthermore, the monochromatic light source is used, so that And obtained at the wavelength of the light source can directly be utilized without use of an approximate expression, as long as birefringent index xcex94n of liquid crystal corresponding to the wavelength is known. Thus, the measurement of d can further be accurately performed without being affected by accuracy of the approximate expression.
More preferably, the angle deriving means is performed by finding a value of the rotational angle at which the reflected light intensity assumes an extreme value. By using the configuration, the rotational angle that satisfies the polarizing plane-maintaining condition can easily be found from a variation of intensity, and thus the measurement can readily be performed.
More preferably, a Jones matrix is used in the xcex94nxc2x7d deriving means. By employing this configuration, the variation of the polarization state can be expressed only by applying a mechanical calculation, and thus the operation for obtaining thickness d can be simplified.
More preferably, using the Jones matrix, in the case that an angle formed by a transmission axis of the first polarizing means and a direction of alignment on an entrance side surface of the liquid crystal layer is assumed to be xcfx86, and that a twist angle of the liquid crystal layer is assumed to be "THgr", a reasonable value of xcex94nxc2x7d/xcfx80 is found from an angle xcfx86 at which a polarizing plane of the reflected light is maintained and from the known twist angle "THgr", to find xcex94nxc2x7d at a wavelength xcex from an obtained value of xcex94nxc2x7d/xcex.
By employing the configuration, unreasonable values of xcex94nxc2x7d/xcex are eliminated such that the most reasonable xcex94nxc2x7d/xcex can be found, and xcex94n and xcex are known, so that d can be found.
More preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are orthogonal to each other. By this configuration, the intensity of the exit light will be a minimal value of 0 at the rotational angle satisfying the polarizing plane maintaining condition, which facilitates finding of the rotational angle.
More preferably, a transmission axis of the first polarizing means and a transmission axis of the second polarizing means are parallel to each other. By this configuration, the intensity of the exit light will be a maximal value at the rotational angle satisfying the polarizing plane-maintaining condition, which facilitates finding of the rotational angle.
More preferably, the reflection region has a diffusibility and light is received by the light-receiving means at a position off a positive reflection direction corresponding to the entering. By employing this configuration, a positive reflection component from an upper substrate can be excluded, so that the thickness measurement can be performed also for the reflection type liquid crystal cell using the reflector having diffusibility, without any effect caused by the positive reflection component from the upper substrate.
Further, to achieve the object described above, a device for deriving a wavelength according to the present invention includes a light source; a first polarizing means for transmitting light from the light source; a second polarizing means for transmitting reflected light reflected at an object to be measured; a light-receiving means for receiving the reflected light transmitted through the second polarizing means; a dispersing means for spectrally resolving the reflected light received by the light-receiving means to detect a relation between a wavelength xcex and a reflected light intensity; and a wavelength deriving means for finding a wavelength satisfying a polarizing plane-maintaining condition in that the reflected light returns maintaining a same polarizing plane as a polarizing plane at the time of the entering, i.e., that a difference in optical path lengths between an ordinary ray and an extraordinary ray of the reflected light is a sum of an integer multiple of the wavelength and a half-wavelength, or an integer multiple of a wavelength.
By using the configuration described above, the reflected light is utilized rather than transmitted light, so that an accurate measurement can also be performed for a reflection-type liquid crystal cell. Further, detection is performed for a change of the polarization state, so that the wavelength can be derived irrespective of the property of the reflection region, such as being specular and diffusible.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.